clocksource: keystone: add bindings for keystone timer

clocksource: keystone: add bindings for keystone timer

This patch provides bindings for the 64-bit timer in the KeyStone
architecture devices. The timer can be configured as a general-purpose 64-bit
timer, dual general-purpose 32-bit timers. When configured as dual 32-bit
timers, each half can operate in conjunction (chain mode) or independently
(unchained mode) of each other.

It is global timer is a free running up-counter and can generate interrupt
when the counter reaches preset counter values.

Documentation:
http://www.ti.com/lit/ug/sprugv5a/sprugv5a.pdf

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>